1. Field of the Invention
The present invention relates generally to optical fiber design and more particularly an optical fiber having a very large mode field core.
2. Background of the Related Art
High-power single-mode fiber lasers having output power in the range of 1-50 kW are now coming into widespread use in the industrial fields of welding, high-speed cutting, brazing, and drilling. Fiber lasers have high wall plug power efficiency, and very good beam characteristics. The beam from fiber lasers can be focused to small spot sizes with long focal length lenses with consistent beam properties independent of power level or pulse duration. Ytterbium single-mode fiber lasers with an M2 of 1.1 have continually increased in power to the multi-kW level, and can be focused to 10-15 μm spot diameters with perfect Gaussian distribution. Further increasing power will open up additional markets in the future.
However, a major limitation to the application of high-power fiber lasers in industrial applications is power loss due to non-linear effects as the beam propagates through the delivery fiber from the fiber laser source to the work area. In most instances, the high power beam generated at the laser source must travel through 10-50 meters of delivery fiber to the work area. It is through this delivery fiber that the system can experience significant power loss due to non-linear effects within the current delivery fiber designs.
All optical fibers experience some signal loss due to attenuation and non-linearities within the fiber itself. Minimizing the effect of these imperfections is critical to maximizing the output power of the laser. To attain higher output power, it is desirable to use optical fibers with a large effective mode area while maintaining single mode guidance. Due to the reduced optical intensities, such fibers effectively have lower non-linearities and a higher damage threshold, which makes them suitable for such applications as the amplification of intense pulses or for single frequency signals, for example.
Conventional single mode fibers can in theory be adapted to provide a large effective mode area. To obtain single-mode guidance despite a large mode area, the numerical aperture of the optical fiber must be decreased, i.e., the refractive index difference between the core and the cladding must be reduced. However, as the numerical aperture decreases the guidance of the fiber weakens and significant losses can arise from small imperfections of the fiber or from bending. Moreover, the fiber may no longer strictly propagate in single-mode, as some higher-order modes may also propagate with relatively small losses. To minimize multi-mode propagation and strengthen the guidance of the fiber, specially optimized refractive index profiles are used, which allow a somewhat better compromise between robust guidance and large mode area. Nevertheless, large mode area single mode fibers have typically been limited to an effective mode area of about 615 μm2 (28 μm mode field diameter).
Large mode area fibers can also be created using photonic crystal fibers (PCFs). Photonic crystal fiber (PCF) (also called holey fiber or microstructure fiber) is an optical fiber, which derives its waveguide properties not from a spatially varying material composition, but from an arrangement of very tiny air holes, which extend longitudinally in a symmetric pattern through the whole length of fiber. Such air holes can be obtained by creating a fiber preform with holes made by stacking capillary tubes (stacked tube technique). Soft glasses and polymers also allow the fabrication of preforms for PCFs by extrusion. There is a great variety of hole arrangements, leading to PCFs with very different properties. A typical PCF has a regular array of hexagonally placed air holes surrounding a solid core, which supports guided modes in the solid core by providing a composite cladding consisting of regular air holes in a glass background, the air holes having a lower effective refractive index than that of the core. To reduce the number of guided modes, the state-of-the-art PCF designs employ small air holes with a hole-diameter-to-pitch ratio d/A of less than 0.1. In this regime, the PCF is very weakly guiding, leading to a high degree of environmental sensitivity. As a result, robust single-mode propagation in PCFs has also been limited to a MFD of approximately 28 μm, a level similar to that of conventional fiber, which is not surprising considering the similarity in the principle behind the two approaches.
More recent PCF designs have exploited a cladding formed not by a large number of smaller holes, but rather by a limited number of large air holes. The design comprises a solid core surrounded by a ring of very few large air holes with an equivalent hole-diameter-to pitch ratio, d/Λ, larger than 0.7. This large hole cladding PCF design has been demonstrated to provide effective mode areas of up to 1400 μm2 (42 μm effective core diameter). This is about 2.5 times higher than for ordinary single-mode fibers or conventional small hole PCF's.
Despite the significant progress made in optical fiber design, further improvement is still required to fully take advantage of the very high power lasers (25 kW to 50 kW) currently available and even higher power designs being developed, as well as to improve telecommunications devices. While the emphasis hereinabove is concentrated on large mode field fibers for industrial fiber laser applications, there is also a need in the telecommunication industry for large mode field active fibers for use in fiber lasers and fiber amplifiers. The large mode area allows the active fiber to provide improved amplification over shorter lengths.
There is thus a defined need for single mode holding large mode area fibers that are less susceptible to damage and that are more efficient at propagating a single spatial mode.